In an air-conditioning apparatus such as a multi-air-conditioning apparatus for a building, a refrigerant is circulated between an outdoor unit, which is a heat source unit disposed, for example, outside a building, and indoor units disposed in rooms in the building. The refrigerant transfers heat or removes heat to heat or cool air, thus heating or cooling a conditioned space through the heated or cooled air. Hydrofluorocarbon (HFC) refrigerants are often used as the refrigerant, for example. Air-conditioning apparatuses that use natural refrigerants as the refrigerant, such as carbon dioxide (CO2), have also been proposed.
In an air-conditioning apparatus called a chiller, a heat source unit disposed outside a building generates cooling energy or heating energy. Water, antifreeze, or the like is heated or cooled by a heat exchanger disposed in an outdoor unit and it is carried to an indoor unit, such as a fan coil unit or a panel heater, to perform heating or cooling (refer to Patent Literature 1, for example).
With another type of air-conditioning apparatus, called an exhaust-heat recovery chiller, four water pipes are connected between the heat source unit and the indoor unit, and cooled water and heated water, for example, are simultaneously supplied so that either cooling or heating can be freely selected by the indoor unit (see Patent Literature 2, for example).
Another type of air-conditioning apparatus is structured so that a heat exchanger for a primary refrigerant and a secondary refrigerant is disposed near each indoor unit, and the secondary refrigerant is carried to the indoor unit (see Patent Literature 3, for example).
Another type of air-conditioning apparatus is structured so that the outdoor unit and a branch unit having heat exchangers are connected with two pipes so that the secondary refrigerant is carried to the indoor unit (see Patent Literature 4, for example).